1. Field of the Invention
The present invention relates in general to a coating composition for forming a coating film on a granular fertilizer in order to delay the release of plant nutrient elements therefrom and a method of coating a granular fertilizer with such a coating film.
2. Description of the Prior Art
Inorganic synthetic fertilizers are generally water-soluble, and thus readily available to plants. Thus, it is difficult to maintain the effect of the fertilizers for a relatively long period due to the loss of nutrients of the fertilizers through leaching, soil erosion, denitrification or the like. Furthermore, due to too ready availability of the fertilizers, crops tend to be damaged when a soil solution containing a relatively large amount of dissolved nutrients is brought into contact with the crop roots. Therefore, slow-release fertilizers have been proposed to slow down the rate of release of nutrients from the fertilizers. For this purpose, for example, it has been proposed to coat granular fertilizers with organic or inorganic films. Majority of the films are organic ones which are made of resin or the like. An organic solvent is generally used in the process to coat a granular fertilizer with an organic film. However, the use of organic solvents is not desirable from the environmental point of view because they are toxic and inflammable.
There is a proposed method of producing a granular fertilizer coated with a resin film without using an organic solvent. In this method, granules of the fertilizer are coated with an aqueous dispersion of a resin, and then the coated granules are dried to form the resin film on the granular fertilizer. However, the resin film on the granular fertilizer has the following drawbacks.
It is inferior in strength and gas permeability. Blocking, i.e. undesirable adhesion of the resin film, tends to occur. Furthermore, it is necessary to spend relatively much time and energy to dry the aqueous resin film because latent heat of vaporization of water is relatively large.